Method of making coextruded, crosslinked polyolefin foam with kee cap layers

ABSTRACT

The present disclosure is directed to a physically crosslinked, closed cell continuous multilayer foam structure comprising at least one foam polypropylene/polyethylene layer with a KEE cap layer. The multilayer foam structure can be obtained by coextruding a multilayer structure comprising at least one foam layer composition layer with at least one cap layer composition layer, irradiating the coextruded structure with ionizing radiation, and continuously foaming the irradiated structure.

FIELD OF THE INVENTION

This disclosure relates to multilayer foam structures with a KEE(ketone-ethylene-ester) terpolymer cap layer. More particularly, thisdisclosure relates to a physically crosslinked, closed cell continuousmultilayer foam structure comprising a KEE cap layer.

BACKGROUND OF THE INVENTION

Polyolefin foams can be used in various applications including, but notlimited to, a trim component in a vehicle interior, a roofing membrane,and a flooring underlayment. When used in these various applications,the polyolefin foam can be bonded to polyvinyl chloride (PVC) skins orpolyvinyl chloride foams. Traditionally, to obtain satisfactory adhesionbetween polyolefin foam and polyvinyl chloride, a surface modificationtreatment such as corona, plasma, or chemical was applied to thepolyolefin foam. Afterwards, an adhesion primer and/or an adhesivewere/was applied to the surface modified polyolefin foam to obtainsatisfactory adhesion.

However, modifying the surface of the polyolefin foam followed bycoating the foam with a primer and/or adhesive are additional processingsteps between the manufacturing of the foam and the final application.This can add cost which may render the process uneconomical forcommercial purposes. In addition, polyolefin surfacemodification—particularly with corona—can also be temporary and may notbe suitable for instances where the treated foam is stored in awarehouse or a retail store for an extended period of time.

SUMMARY OF THE INVENTION

It has been discovered that coextruding a polyolefin foam layercomposition with a KEE (ketone-ethylene-ester) terpolymer cap layer canovercome the issues associated with both (a) treating the surface of apolyolefin foam with corona, plasma, or a chemical to modify the foamsurface: and (b) applying an adhesion primer and/or an adhesive to thesurface modified polyolefin foam. Since KEE is highly miscible in (andthus highly compatible with) PVC, a coextruded polyolefin foam with aKEE cap layer is expected to readily heat bond to PVC without the needfor a surface treatment, a primer, and/or an adhesive. In addition,unlike corona, the KEE cap layer does not have a “shelf life” whereextended periods of time in a warehouse or retail store can render theproduct less susceptible to adhesion to a primer or adhesive.

In some embodiments, a method of forming a multilayer structure isprovided, the method comprising: coextruding a first layer includingpolypropylene, polyethylene, or a combination of polypropylene andpolyethylene and a chemical foaming agent, and a second layer on a sideof the first layer, the second layer including at least 15 wt. %ketone-ethylene-ester terpolymer and polypropylene, polyethylene, or acombination of polypropylene and polyethylene; irradiating thecoextruded layers with ionizing radiation and foaming the irradiated,coextruded layers. In some embodiments, the first layer comprises 2-15wt. % KEE. In some embodiments, the first layer comprises at least 70wt. % polypropylene, polyethylene, or a combination of polypropylene andpolyethylene. In some embodiments, the first layer comprises additivesin an amount of 1-20 wt. %. In some embodiments, the second layercomprises additives in an amount of 1-8 wt. %. In some embodiments, thepolypropylene has a melt flow index of 0.1-25 grams per 10 minutes at230° C. In some embodiments, the polyethylene has a melt flow index of0.1-25 grams per 10 minutes at 190° C. In some embodiments, the amountof chemical foaming agent in the first layer is 4-10 wt. %. In someembodiments, the chemical foaming agent comprises azodicarbonamide. Insome embodiments, the first layer comprises a crosslinking agent. Insome embodiments, the amount of crosslinking agent in the first layer is1-3 wt. %. In some embodiments, the ionizing radiation is selecting fromthe group consisting of alpha, beta (electron beams), x-ray, gamma, andneutron. In some embodiments, the coextruded structure is irradiated upto four separate times. In some embodiments, the ionizing radiation isan electron beam with an acceleration voltage of 200-1500 kV. In someembodiments, an absorbed electron beam dosage is 10-500 kGy. In someembodiments, the ionizing radiation crosslinks the extruded structure toa crosslinking degree of 20-75%. In some embodiments, foaming comprisesheating the irradiated structure with molten salt and radiant heaters ora hot air oven. In some embodiments, the multilayer foam structure has adensity of 20-250 kg/m³. In some embodiments, the multilayer foamstructure has an average closed cell size of 0.05-1.0 mm. In someembodiments, the multilayer foam structure has a thickness of 0.2-50 mm.

Reference to “about” a value or parameter herein includes (anddescribes) variations that are directed to that value or parameter perse. For example, description referring to “about X” includes descriptionof “X”. In addition, reference to phrases “less than”, “greater than”,“at most”, “at least”, “less than or equal to”, “greater than or equalto”, or other similar phrases followed by a string of values orparameters is meant to apply the phrase to each value or parameter inthe string of values or parameters. For example, a statement that thelayer has less than about 20 wt. %, about 15 wt. %, or about 10 wt. % ofa chemical foaming agent, is meant to mean that the weight percentage ofthe chemical foaming agent in the layer can be less than about 20 wt. %,less than about 15 wt. %, or less than about 10 wt. %.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It is also to be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It is further to beunderstood that the terms “includes, “including,” “comprises,” and/or“comprising,” when used herein, specify the presence of stated features,integers, steps, operations, elements, components, and/or units but donot preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, units, and/or groupsthereof.

It is understood that aspects and embodiments described herein include“consisting” and/or “consisting essentially of” aspects and embodiments.For all methods, systems, compositions, and devices described herein,the methods, systems, compositions, and devices can either comprise thelisted components or steps, or can “consist of” or “consist essentiallyof” the listed components or steps. When a system, composition, ordevice is described as “consisting essentially of” the listedcomponents, the system, composition, or device contains the componentslisted, and may contain other components which do not substantiallyaffect the performance of the system, composition, or device, but eitherdo not contain any other components which substantially affect theperformance of the system, composition, or device other than thosecomponents expressly listed; or do not contain a sufficientconcentration or amount of the extra components to substantially affectthe performance of the system, composition, or device. When a method isdescribed as “consisting essentially of” the listed steps, the methodcontains the steps listed, and may contain other steps that do notsubstantially affect the outcome of the method, but the method does notcontain any other steps which substantially affect the outcome of themethod other than those steps expressly listed.

In the disclosure, “substantially free of” a specific component, aspecific composition, a specific compound, or a specific ingredient invarious embodiments, is meant that less than about 5%, less than about2%, less than about 1%, less than about 0.5%, less than about 0.1%, lessthan about 0.05%, less than about 0.025%, or less than about 0.01% ofthe specific component, the specific composition, the specific compound,or the specific ingredient is present by weight. Preferably,“substantially free of” a specific component, a specific composition, aspecific compound, or a specific ingredient indicates that less thanabout 1% of the specific component, the specific composition, thespecific compound, or the specific ingredient is present by weight.

Additional advantages will be readily apparent to those skilled in theart from the following detailed description. The examples anddescriptions herein are to be regarded as illustrative in nature and notrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described with reference to the accompanyingfigures, in which:

FIG. 1A is an image of Example 1A at 30× magnification and 45° fromprimary surface;

FIG. 1B is an image of Example 1B at 30× magnification and 45° fromprimary surface;

FIG. 2A is an image of Example 2A at 30× magnification and 45° fromprimary surface;

FIG. 2B is an image of Example 2B at 30× magnification and 45° fromprimary surface;

FIG. 3 is an image of Example 3 at 30× magnification and 45° fromprimary surface;

FIG. 4A is an image of Example 4A at 30× magnification and 45° fromprimary surface;

FIG. 4B is an image of Example 4B at 30× magnification and 45° fromprimary surface;

FIG. 5 is an image of Example 5 at 30× magnification and 45° fromprimary surface;

FIG. 6 is an image of Example 6 at 30× magnification and 45° fromprimary surface;

FIG. 7 is an image of Example 7 at 30× magnification and 45° fromprimary surface;

FIG. 8 is an image of Example 8 at 30× magnification and 45° fromprimary surface;

FIG. 9 is an image of Example 9 at 30× magnification and 45° fromprimary surface;

FIG. 10 is an image of Example 10 at 30× magnification and 45° fromprimary surface; and

FIG. 11 is an image of Example 11 at 30× magnification and 45° fromprimary surface.

DETAILED DESCRIPTION OF THE INVENTION

Described herein are crosslinked, closed cell coextruded polyolefinfoams with KEE cap layers, as well as methods for producing crosslinked,closed cell coextruded polyolefin foams with KEE cap layers. Inparticular, it has been discovered that coextruding a polyolefin foamlayer composition with a KEE cap layer can overcome the issuesassociated with treating the surface of polyolefin foam with corona,plasma, or a chemical and/or applying an adhesion primer or adhesive tothe surface modified polyolefin film. Specifically, the structuresdisclosed herein can readily bond to polyvinyl chloride films andpolyvinyl chloride foams and can avoid rendering the product less ableto adhere to primers and adhesives as shelf life increases.

The methods for producing a crosslinked, closed cell co-extrudedpolyolefin foam with KEE cap layer(s) may include the steps of: (a)co-extrusion; (b) irradiation; and (c) foaming.

Co-extrusion is the extrusion of multiple layers of materialsimultaneously. This type of extrusion can utilize two or more extrudersto deliver a steady volumetric throughput of material to an extrusionhead (die) which can extrude the materials in the desired form. In theco-extrusion step, compositions can be fed into multiple extruders toform an unfoamed, multilayer structure. For example, an “A” foam layercomposition can be fed into one extruder and a “B” cap layer compositioncan be fed into a second extruder. The method of feeding ingredientsinto the extruders can be based on the design of the extruder and thematerial handling equipment available. Blending ingredients of the foamand cap layer compositions may be performed prior to feeding into theextruders, if necessary, to facilitate their dispersal. A Henshel mixercan be used for such blending. All ingredients can be blended and fedthrough a single port in an extruder. The ingredients can also beindividually fed through separate designated ports for each ingredient.For example, if the crosslinking promoter or any other additive is aliquid, the promoter and/or additives can be added through a feedinggate (or gates) on the extruder or through a vent opening on theextruder (if equipped with a vent) instead of being blended with solidingredients. Combinations of blended ingredients and individualingredient port feeding can also be employed.

Each extruder can deliver a steady amount of each composition into oneor more manifolds. The one or more manifolds may then be fed through asheeting die to create an unfoamed, co-extruded multilayer sheet. Thereare two common methods for co-extruding materials: (1) feed blockmanifolds; and (2) multi-manifolds within the die. Elements of a feedblock manifold can include: (a) inlet ports for upper, middle, and lowerlayers; (b) a streamlined melt lamination area that channels separateflow streams into one laminated melt stream inside the feed block; (c)an adapter plate between the feed block and the sheet die; and/or (d) asheet die (similar to a monolayer die), wherein the laminated meltstream enters the center of the die and spreads out along the manifoldflowing out of the die exit as a distinct multilayer extrudate. Elementsof a multi-manifold die can be: (a) similar to a monolayer die, exceptthat there is more than one feed channel; (b) that each melt channel hasits own choker bar for flow control; and/or (c) that the melt streamsconverge inside the die near the exit and emerge as a distinctmultilayer extrudate.

Layer thicknesses of a multilayer structure provided herein can bedetermined by the design of the manifold(s) and/or die. For example, an80/20 feed block manifold can deliver compositions in approximately a4:1 ratio when the speed and size of each extruder is matchedaccordingly. A 50/50 feed block manifold can deliver compositions inapproximately a 1:1 ratio when the speed and size of each extruder ismatched accordingly. These ratios can be altered by changing, forexample: (a) the amount of material fed into each extruder; (b) therelative extrusion speed between one extruder and another; (c) therelative size of each extruder; and/or (d) the composition (i.e., theviscosity) of the individual layers.

The thickness of the overall multilayer sheet can be controlled by theoverall die gap. However, the overall multilayer sheet thickness canalso be adjusted by stretching (i.e., “drawing”) the melted multilayerextrudate and/or flattening the melted multilayer extrudate through anip.

The multilayer structures disclosed herein can include at least twolayers made up of different compositions, where at least one of thelayers can contain KEE (ketone-ethylene-ester) terpolymer. In addition,the multilayer structures can include at least one layer made up of afoamable or foamed composition. In some embodiments, the multilayerstructures can include at least one “A” polyolefin foam layer and atleast one “B” KEE cap layer. In some embodiments, the “B” KEE cap layercan also include polyolefin. In some embodiments, the “A” polyolefinfoam layer can also include KEE. In some embodiments, the “B” KEE caplayer can also be foamable or foamed.

A foamable composition fed into the extruder can include at least onepolypropylene, at least one polyethylene, and/or a combination thereof.In some embodiments, the foam layer composition can form a polyolefinfoam layer (A) of the multilayered structure. In some embodiments, thefoam layer composition can form a KEE cap layer (B) of the multilayerstructure.

The polypropylene of a foamable composition may contain an elastic orsoftening component. An elastic or softening component is typically anethylene or rubber component and thus includes, but is not limited to,polypropylene, impact modified polypropylene, polypropylene-ethylenecopolymer, impact modified polypropylene-ethylene copolymer, metallocenepolypropylene, metallocene polypropylene-ethylene copolymer, metallocenepolypropylene olefin block copolymer (with a controlled block sequence),polypropylene based polyolefin plastomer, polypropylene based polyolefinelasto-plastomer, polypropylene based polyolefin elastomer,polypropylene based thermoplastic polyolefin blend, and polypropylenebased thermoplastic elastomeric blend. Furthermore, the polypropylenemay be modified with polyether amine.

The polyethylene of a foamable composition includes, but is not limitedto, LDPE, LLDPE (homopolymer, copolymer with butene or hexene or octene,terpolymer with butene and/or hexene and/or octene), VLDPE (homopolymer,copolymer with butene or hexene or octene, terpolymer with butene and/orhexene and/or octene), VLLDPE (homopolymer, copolymer with butene orhexene or octene, terpolymer with butene and/or hexene and/or octene),HDPE, polyethylene-propylene copolymer, metallocene polyethylene,metallocene ethylene-propylene copolymer, and metallocene polyethyleneolefin block copolymer (with a controlled block sequence), any of whichmay contain grafted compatibilizers or copolymers that contain acetateand/or ester groups.

The foam layer composition fed into the extruder can also include atleast one KEE. KEE includes ketone-ethylene-ester terpolymers. Oneexample is ethylene/n-butyl acrylate/carbon monoxide (E/nBA/CO)terpolymer. Other examples include ethylene/vinyl acetate/carbonmonoxide (E/VA/CO) terpolymer and ethylene/2-ethyl hexyl acrylate/carbonmonoxide (E/EHA/CO) terpolymer. These polymers can exhibit various shortchain branching and monomer sequences.

In some embodiments, the amount of at least one polypropylene and/or atleast one polyethylene in a foam layer composition can be greater thanor equal to about 70 PPR %, about 75 PPR %, about 80 PPR %, about 85 PPR%, about 90 PPR %, or about 95 PPR % of the composition. In someembodiments, the amount of at least one polypropylene and/or at leastone polyethylene in a foam layer composition can be less than or equalto about 100 PPR %, about 95 PPR %, about 90 PPR %, about 85 PPR %,about 80 PPR %, or about 75 PPR % of the composition. In someembodiments, the amount of at least one polypropylene and/or at leastone polyethylene in a foam layer composition can be at least about 50wt. %, about 60 wt. %, about 65 wt. %, about 70 wt. %, about 75 wt. %,about 80 wt. %, about 85 wt. %, or about 90 wt. % of the foamcomposition. In some embodiments, the amount of at least onepolypropylene and/or at least one polyethylene in a foam layercomposition can be less than or equal to about 95 wt. %, about 90 wt. %,about 85 wt. %, about 80 wt. %, about 75 wt. %, about 70 wt. %, about 65wt. %, or about 60 wt. % of the foam layer composition. In someembodiments, the amount of at least one polypropylene and/or at leastone polyethylene in a foam layer composition can be about 50-95 wt. %,about 65-90 wt. %, about 70-90 wt. %, or about 70-85 wt. % of the foamlayer composition.

In some embodiments, the amount of KEE can be less than or equal toabout 20 PPR %, about 15 PPR %, about 10 PPR %, or about 5 PPR % of thefoam layer composition. In some embodiments, the amount of KEE can begreater than or equal to about 0.1 PPR %, about 1 PPR %, about 5 PPR %,about 10 PPR %, or about 15 PPR % of the foam layer composition. In someembodiments, the amount of KEE in a foam layer composition can be lessthan or equal to about 20 wt. %, about 15 wt. %, about 10 wt. %, about 5wt. %, or about 1 wt. % of the foam layer composition. In someembodiments, the amount of KEE can be greater than or equal to about 0.1wt. %, about 1 wt. %, about 5 wt. %, about 10 wt. %, or about 15 wt. %of the foam layer composition. In some embodiments, the amount KEE canbe about 0.1-20 wt. %, about 1-15 wt. %, about 2-10 wt. %, or about 3-5wt. % of the foam layer composition.

A cap layer composition fed into the extruder can include at least oneKEE and at least one polypropylene and/or at least one polyethylene (theKEE, polypropylene, and polyethylene as described above). In someembodiments, the cap layer composition can form a KEE cap layer (B) ofthe multilayer structure.

In some embodiments, the amount of KEE in a cap layer composition can begreater than or equal to about 15 PPR %, about 30 PPR %, about 40 PPR %,about 50 PPR %, about 60 PPR %, about 70 PPR %, about 80 PPR %, or about90 PPR % of the cap layer composition. In some embodiments, the amountof KEE in a cap layer composition can be less than or equal to about 100PPR %, about 90 PPR %, about 80 PPR %, about 70 PPR %, about 60 PPR %,about 50 PPR %, about 40 PPR %, or about 30 PPR % of the cap layercomposition. In some embodiments, the KEE in a cap layer composition canbe at least about 15 wt. %, about 30 wt. %, about 40 wt. %, about 50 wt.%, about 60 wt. %, about 70 wt. %, about 80 wt. %, about 90 wt. %, orabout 95 wt. % of the cap layer composition. In some embodiments, theamount of KEE in a cap layer composition can be less than or equal toabout 100 wt. %, about 95 wt. %, about 90 wt. %, about 80 wt. %, about70 wt. %, about 60 wt. %, about 50 wt. %, about 40 wt. %, or about 30wt. % of the cap layer composition. In some embodiments, the KEE in acap layer composition can be about 15-85 wt. %, about 30-70 wt. %, orabout 40-60 wt. % of the cap layer composition.

In some embodiments, the amount of at least one polypropylene and/or atleast one polyethylene in a cap layer composition can be less than orequal to about 85 PPR %, about 70 PPR %, about 60 PPR %, about 50 PPR %,about 40 PPR %, about 30 PPR %, about 20 PPR %, about 15 PPR %, or about10 PPR % of the cap layer composition. In some embodiments, the amountof at least one polypropylene and/or at least one polyethylene in a caplayer composition can be greater than or equal to about 5 PPR %, about10 PPR %, about 15 PPR %, about 20 PPR %, about 30 PPR %, about 40 PPR%, about 50 PPR %, about 60 PPR %, or about 70 PPR % of the cap layercomposition. In some embodiments, the amount of at least onepolypropylene and/or at least one polyethylene in a cap layercomposition can be less than or equal to about 85 wt. %, about 70 wt. %,about 60 wt. %, about 50 wt. %, about 40 wt. %, about 30 wt. %, about 20wt. %, about 15 wt. %, or about 10 wt. % of the cap layer composition.In some embodiments, the amount of at least one polypropylene and/or atleast one polyethylene in a cap layer composition can be greater than orequal to about 5 wt. %, about 10 wt. %, about 15 wt. %, about 20 wt. %,about 30 wt. %, about 40 wt. %, about 50 wt. %, about 60 wt. %, or about70 wt. % of the cap layer composition. In some embodiments, the amountof at least one polypropylene and/or at least one polyethylene in a caplayer composition can be about 15-85 wt. %, about 30-70 wt. %, or about40-60 wt. % of the cap layer composition.

Since a broad range of multilayer structures and foam articles can becreated with the disclosed compositions, a broad range ofpolypropylenes, polyethylenes, and KEE can be employed in thecompositions to meet various in-process manufacturing requirements andcommercial end use requirements.

Non-limiting examples of commercially available ketone-ethylene-ester(KEE) terpolymers include, but are not limited to, the ELVALOY® HPSeries (E/nBa/Co terpolymers) from the Dow Chemical Company (e.g.,ELVALOY® HP441, ELVALOY® HP641, etc.) and ELVALOY® 741, ELVALOY® 742,and ELVALOY® 4924 (E/VA/CO terpolymers) also from the Dow ChemicalCompany.

A non-limiting example of “polypropylene” is an isotactichomopolypropylene. Commercially available examples include, but are notlimited to, FF018F from Braskem, 3271 from Total Petrochemicals, andCOPYLENE™ CH020 from Conoco.

A non-limiting example of an “impact modified polypropylene” is ahomopolypropylene with ethylene-propylene (EP) copolymer rubber. Therubber can be amorphous or semicrystalline but is not in sufficientquantities to render the material any plastomeric or elastomericproperties. A few non-limiting examples of commercially available“impact modified polypropylene” are TI4003F and TI4015F from Braskem,and PRO-FAX® 8623 and PRO-FAX® SB786 from LyondellBasell.

“Polypropylene-ethylene copolymer” is polypropylene with random ethyleneunits. A few non-limiting examples of commercially available“polypropylene-ethylene copolymer” are 6232, 7250FL, and Z9421 fromTotal Petrochemicals, 6D20 and DS6D81 from Braskem, and PRO-FAX® RP311Hand ADSYL™ 7415 XCP from LyondellBasell.

“Impact modified polypropylene-ethylene copolymer” is polypropylene withrandom ethylene units and with ethylene-propylene (EP) copolymer rubber.The rubber can be amorphous or semicrystalline, but is not in sufficientquantities to render the material any plastomeric or elastomericproperties. A non-limiting example of a commercially available impactmodified polypropylene-ethylene copolymer is PRISMA® 6910 from Braskem.

“Metallocene polypropylene” is metallocene syndiotactichomopolypropylene, metallocene atactic homopolypropylene, andmetallocene isotactic homopolypropylene. Non-limiting examples of“metallocene polypropylene” are those commercially available under thetrade names METOCENE™ from LyondellBasell and ACHIEVE™ from ExxonMobil.Metallocene polypropylenes are also commercially available from TotalPetrochemicals and include, but are not limited to, grades M3551,M3282MZ, M7672, 1251, 1471, 1571, and 1751.

“Metallocene polypropylene-ethylene copolymer” is metallocenesyndiotactic, metallocene atactic, and metallocene isotacticpolypropylene with random ethylene units. Commercially availableexamples include, but are not limited to, Lumicene® MR10MX0 andLumicene® MR60MC2 from Total Petrochemicals and Purell® SM170G fromLyondellBasell.

“Metallocene polypropylene olefin block copolymer” is a polypropylenewith alternating crystallizable hard “blocks” and amorphous soft“blocks” that are not randomly distributed—that is, with a controlledblock sequence. An example of “metallocene polypropylene olefin blockcopolymer” includes, but is not limited to, the INTUNE™ product linefrom the Dow Chemical Company.

“Polypropylene based polyolefin plastomer” (POP) and “polypropylenebased polyolefin elastoplastomer” are both metallocene andnon-metallocene propylene based copolymers with plastomeric andelastoplastomeric properties. Non-limiting examples are thosecommercially available under the trade name VERSIFY™ (metallocene) fromthe Dow Chemical Company, VISTAMAXX™ (metallocene) from ExxonMobil, andKOATTRO™ (non-metallocene) from LyondellBasell (a butene-1 based line ofplastomeric polymers—certain grades are butene-1 homopolymer based andothers are polypropylene-butene-1 copolymer based materials).

“Polypropylene based polyolefin elastomer” (POE) is both metallocene andnon-metallocene propylene based copolymer with elastomeric properties.Non-limiting examples of propylene based polyolefin elastomers are thosepolymers commercially available under the trade names VERSIFY™(metallocene) from the Dow Chemical Company and VISTAMAXX™ (metallocene)from ExxonMobil.

“Polypropylene based thermoplastic polyolefin blend” (TPO) ispolypropylene, polypropylene-ethylene copolymer, metallocenehomopolypropylene, and metallocene polypropylene-ethylene copolymer,which have ethylene-propylene copolymer rubber in amounts great enoughto give the thermoplastic polyolefin blend (TPO) plastomeric,elastoplastomeric or elastomeric properties. Non-limiting examples ofpolypropylene based polyolefin blend polymers are those polymer blendscommercially available under the trade names EXCELINK™ from JSRCorporation, THERMORUN™ and ZELAS™ from Mitsubishi Chemical Corporation,ADFLEX™ and SOFTELL™ from LyondellBasell, and TELCAR™ from Teknor ApexCompany.

“Polypropylene based thermoplastic elastomer blend” (TPE) ispolypropylene, polypropylene-ethylene copolymer, metallocenehomopolypropylene, and metallocene polypropylene-ethylene copolymer,which have diblock or multiblock thermoplastic rubber modifiers (SEBS,SEPS, SEEPS, SEP, SERC, CEBC, HSB and the like) in amounts great enoughto give the thermoplastic elastomer blend (TPE) plastomeric,elastoplastomeric, or elastomeric properties. Non-limiting examples ofpolypropylene based thermoplastic elastomer blend polymers are thosepolymer blends commercially available under the trade name GLS™DYNAFLEX™ and GLS™ VERSAFLEX™ from Polyone Corporation, MONPRENE® fromTeknor Apex Company, and DURAGRIP® from A. Schulman.

“VLDPE” and “VLLDPE” are very low density polyethylene and very linearlow density polyethylene containing an elastic or softening component,typically α-olefins of butene and/or hexene and/or octene. Non-limitingexamples of VLDPE and VLLDPE are commercially available under thetradename FLEXOMER™ from the Dow Chemical Company and particular gradesof STAMYLEX™ from Borealis.

“LDPE” and “LLDPE” are low density polyethylene and linear low densitypolyethylene, respectively. Non-limiting examples of LDPE and LLDPEinclude at least those provided by ExxonMobil™ (e.g., LLP8501.67) andthe Dow Chemical Company (e.g., DFDA-7059 NT 7). Commercial LLDPEpolymers are typically copolymers containing α-olefins of butene and/orhexene and/or octane.

“Metallocene polyethylene” is metallocene based polyethylene withproperties ranging from non-elastic to elastomeric. Non-limitingexamples of metallocene polyethylene are commercially available underthe trade name ENGAGE™ from the Dow Chemical Company Chemical Company,ENABLE™ and EXCEED™ from ExxonMobil™, and QUEO™ from Borealis.

“Metallocene polyethylene olefin block copolymer” is a polyethylene withalternating crystallizable hard “blocks” and amorphous soft “blocks”that are not randomly distributed—that is, with a controlled blocksequence. An example of “metallocene polyethylene olefin blockcopolymer” includes, but is not limited to, the INFUSE™ product linefrom the Dow Chemical Company Chemical Company.

These polyethylenes may also be copolymers and terpolymers containingacetate and/or ester groups. The comonomer groups include, but are notlimited to, vinyl acetate, methyl acrylate, ethyl acrylate, butylacrylate, glycidyl methacrylate, and acrylic acid. Non-limiting examplesare commercially available under the tradename BYNEL®, ELVAX® andELVALOY® from The Dow Chemical Company; EVATANE®, LOTADER®, and LOTRYL®from Arkema; ESCORENE™, ESCOR™, and OPTEMA™ from ExxonMobil.

The polypropylenes and polyethylenes listed above can be functionalized.Functionalized polypropylenes and polyethylenes include a graftedmonomer. Typically, the monomer has been grafted to the polypropylene orpolyethylene by a free radical reaction. Suitable monomers for preparingfunctionalized polypropylenes and polyethylenes are, for example,olefinically unsaturated monocarboxylic acids, e.g. acrylic acid ormethacrylic acid, and the corresponding tert-butyl esters, e.g.tert-butyl (meth) acrylate, olefinically unsaturated dicarboxylic acids,e.g. fumaric acid, maleic acid, and itaconic acid and the correspondingmono- and/or di-tert-butyl esters, e.g. mono- or di-tert-butyl fumarateand mono- or di-tert-butyl maleate, olefinically unsaturateddicarboxylic anhydrides, e.g. maleic anhydride, sulfo- orsulfonyl-containing olefinically unsaturated monomers, e.g.p-styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropenesulfonic acidor 2-sulfonyl-(meth)acrylate, oxazolinyl-containing olefinicallyunsaturated monomers, e.g. vinyloxazolines and vinyloxazolinederivatives, and epoxy-containing olefinically unsaturated monomers,e.g. glycidyl (meth)acrylate or allyl glycidyl ether.

The most commonly commercially available functionalized polypropylenesare the ones functionalized with maleic anhydride. Non-limiting examplesare the ADMER® QF and QB Series from Mitsui Chemicals, the PLEXAR® 6000Series from LyondellBasell, the BYNEL® 5000 Series from the Dow ChemicalCompany, and the OREVAC® PP Series from Arkema.

The most commonly commercially available functionalized polyethylenesare also those functionalized with maleic anhydride. Non-limitingexamples are the ADMER® NF and SE Series from Mitsui Chemicals, thePLEXAR® 1000, 2000, and 3000 Series from LyondellBasell, the BYNEL®2100, 3000, 3800, 3900, 4000 Series from the Dow Chemical Company, andthe OREVAC® PE, T, and some of the LOTADER® Series from Arkema.

Polyethylenes functionalized with other grafted monomers are alsocommercially available. Non-limiting examples include the BYNEL® 1100,2200, and 3100 Series from the Dow Chemical Company and the LOTADER® AXSeries from Arkema.

Note that polymers other than polypropylene and polyethylenefunctionalized with maleic anhydride are also commercially available.For example, the ROYALTUF® Series from Addivant are a series of EPDMrubbers functionalized with maleic anhydride. In another example, theKRATON™ FG series from Kraton are a series of SEBS polymersfunctionalized with maleic anhydride.

The composition of any foamable layer and any cap layer provided hereincan contain at least one polypropylene having a melt flow index fromabout 0.1 to about 25 grams per 10 minutes at 230° C. and/or at leastone polyethylene having a melt flow index from about 0.1 to about 25grams per 10 minutes at 190° C. In some embodiments, the melt flow indexof the polypropylene(s) and/or polyethylene(s) is preferably from about0.3 to about 20 grams per 10 minutes at 230° C. and at 190° C.,respectively, and more preferably from about 0.5 to about 15 grams per10 minutes at 230° C. and at 190° C., respectively. The “melt flowindex” (MFI) value for a polymer is defined and measured according toASTM D1238 at 230° C. for polypropylenes and polypropylene basedmaterials and at 190° C. for polyethylenes and polyethylene basedmaterials using a 2.16 kg plunger for 10 minutes. The test time may bereduced for relatively high melt flow resins.

The MFI can provide a measure of flow characteristics of a polymer andis an indication of the molecular weight and processability of a polymermaterial. High MFI values correspond to low viscosities. If the MFIvalues are too high, extrusion according to the present disclosurecannot be satisfactorily carried out. Problems associated with MFIvalues that are too high include low pressures during extrusion,problems setting the thickness profile, uneven cooling profile due tolow melt viscosity, poor melt strength, and/or machine problems.Conversely, low MFI values correspond to high viscosities. MFI valuesthat are too low can cause high pressures during melt processing, sheetquality and profile problems, and higher extrusion temperatures whichcause a risk of foaming agent decomposition and activation.

The above MFI ranges are also important for foaming processes becausethey can reflect the viscosity of the material, which has an effect onthe foaming. Without being bound by any theory, it is believed there areseveral reasons why particular MFI values are more effective. A lowerMFI material may improve some physical properties as the molecular chainlength is greater, creating more energy needed for chains to flow when astress is applied. Also, the longer the molecular chain (MW), the morecrystal entities the chain can crystallize, thus providing more strengththrough intermolecular ties. However, at too low an MFI, the viscositybecomes too high. On the other hand, polymers with higher MFI valueshave shorter chains. Therefore, in a given volume of a material withhigher MFI values, there are more chain ends on a microscopic levelrelative to polymers having a lower MFI, which can rotate and createfree volume due to the space needed for such rotation (e.g., rotationoccurring above the T_(g), or glass transition temperature of thepolymer). This can increase the free volume and enables an easy flowunder stress forces.

In addition to the polymers, the compositions fed into the extruders mayalso contain additives compatible with producing the disclosedmultilayered structures. Common additives include, but are not limitedto, organic peroxides, antioxidants, lubricants, processing aids,thermal stabilizers, colorants, flame retardants, antistatic agents,nucleating agents, plasticizers, antimicrobials, fungicides, lightstabilizers, UV absorbents, anti-blocking agents, fillers, deodorizers,odor adsorbers, anti-fogging agents, volatile organic compound (VOC)adsorbers, semi-volatile organic compound (SVOC) adsorbers, thickeners,cell size stabilizers, metal deactivators, and combinations thereof.Suitable antioxidant packages include PR023, a blend of standardantioxidants developed by Toray Industries, Inc. and compounded byTechmer PM and/or PR086, a blend of antioxidants formulated by TorayPlastics (America), Inc. and blended by Amfine Chemical. A suitableprocessing aid may include TPM11166 from Techmer PM. An additionalexample of a suitable additive may include PM91399, which is a blackconcentrate from Techmer PM.

In some embodiments, the amount of additive(s) other than the chemicalfoaming agent(s) and the crosslinking promoter(s) in a foam layercomposition and/or a cap layer composition can be less than or equal toabout 20 PPR %, about 15 PPR %, about 10 PPR %, about 8 PPR %, about 6PPR %, about 4 PPR %, about 2 PPR %, or about 1 PPR % of thecomposition. In some embodiments, the amount of additive(s) other thanthe chemical foaming agent(s) and the crosslinking promoter(s) in a foamlayer composition and/or a cap layer composition can be greater than orequal to about 0.5 PPR %, about 1 PPR %, about 2 PPR %, about 4 PPR %,about 6 PPR %, about 8 PPR %, about 10 PPR %, or about 15 PPR % of thecomposition. In some embodiments, the amount of additive(s) other thanthe chemical foaming agent(s) and the crosslinking promoter(s) in a foamlayer composition and/or a cap layer composition can be about 0.5-20 PPR%, about 1-10 PPR %, about 1.5-5 PPR %, or about 2-3 PPR % of thecomposition. In some embodiments, the amount of additive(s) other thanthe chemical foaming agent(s) and the crosslinking promoter(s) in a foamlayer composition can be about 1-20 wt. %, about 3-15 wt. %, about 5-10wt. %, or about 6-8 wt. % of the foam layer composition. In someembodiments, the amount of additive(s) other than the chemical foamingagent(s) and the crosslinking promoter(s) in a cap layer composition canbe about 0.5-10 wt. %, about 1-6 wt. %, or about 1.5-5 wt. % of the caplayer composition.

Regardless of how ingredients are fed into the extruders, the shearingforce and mixing within an extruder can be sufficient to produce ahomogenous layer. Co-rotating and counter-rotating twin screw extruderscan provide sufficient shearing force and mixing thru the extruderbarrel to extrude a layer with uniform properties.

Specific energy is an indicator of how much work is being applied duringthe extrusion of the ingredients for a layer and how intensive theextrusion process is. Specific energy is defined as the energy appliedto a material being processed by the extruder, normalized to a perkilogram basis. The specific energy is quantified in units of kilowattsof applied energy per total material fed in kilograms per hour. Specificenergy is calculated according to the formula:

${\text{Specific~~Energy} = \frac{{KW}( \text{applied} )}{\text{feedrate}( \frac{kg}{hr} )}},\text{where}$${{KW}( \text{applied} )} = \frac{\begin{matrix}{{KW}\mspace{14mu} ( \text{motor~~rating} )*} \\{( {\% \mspace{14mu} {torque}\mspace{14mu} {from}\mspace{14mu} {maximum}\mspace{14mu} {allowable}\mspace{14mu} {in}\mspace{14mu} {decimal}\mspace{14mu} {form}} )*} \\{{RPM}\mspace{14mu} ( {{actual}\mspace{14mu} {running}\mspace{14mu} {RPM}} )*0.97\mspace{14mu} ( {{gearbox}\mspace{14mu} {efficiency}} )}\end{matrix}}{{Max}\mspace{14mu} {RPM}\mspace{14mu} ( \text{capability~~of~~extruder} )}$

Specific energy can be used to quantify the amount of shearing andmixing of the ingredients within the extruder. The extruders used toform the multilayer structures disclosed herein can be capable ofproducing a specific energy of at least about 0.100 kW·hr/kg, preferablyat least about 0.120 kW·hr/kg, and more preferably at least about 0.150kW·hr/kg.

Any foamable layer can contain a chemical foaming agent (CFA). Theextrusion temperature for any foamable layer can be at least 10° C.below the thermal decomposition initiation temperature of the chemicalfoaming agent. If the extrusion temperature exceeds the thermaldecomposition temperature of the foaming agent, then the foaming agentwill decompose, resulting in undesirable “prefoaming.” The extrusiontemperature for any cap layer can be at least 10° C. below the thermaldecomposition initiation temperature of the chemical foaming agent inany foamable layer adjacent to the cap layer. If the extrusiontemperature of the cap layer exceeds the thermal decompositiontemperature of the foaming agent in the adjacent layer, then the foamingagent in the adjacent layer can decompose, also resulting in undesirable“prefoaming.”

The foam layer composition can include a variety of different chemicalfoaming agents. Examples of chemical foaming agents include, but are notlimited to, azo compounds, hydrazine compounds, carbazides, tetrazoles,nitroso compounds, and carbonates. In addition, a chemical foaming agentmay be employed alone or in any combination. One chemical foaming agentthat can be used in some embodiments is azodicarbonamide (ADCA). Anexample of a chemical foaming agent is P.T. Lauten Otsuka Chemical'sUNIFOAM® TC-181, which is imported by Biddle Sawyer Corporation and soldin the United States as Azofoam® TC-181. ADCA's thermal decompositiontypically occurs at temperatures between about 190 to 230° C. In orderto prevent ADCA from thermally decomposing in the extruder, extrudingtemperature can be maintained at or below 190° C.

The amount of chemical foaming agent in a foam layer composition and/ora cap layer composition can be less than or equal to about 30 PPR %about 20 PPR %, about 15 PPR %, about 10 PPR %, about 8 PPR %, or about5 PPR % of the composition. In some embodiments, the amount of chemicalfoaming agent in a foam layer composition and/or a cap layer compositioncan be greater than or equal to about 1 PPR %, about 5 PPR %, about 10PPR %, about 15 PPR %, or about 20 PPR % of the composition. In someembodiments, the amount of chemical foaming agent in a foam layercomposition and/or a cap layer composition can be about 1-30 PPR %,about 2-20 PPR %, about 5-15 PPR %, or about 6-10 PPR % of thecomposition. In some embodiments, the amount of chemical foaming agentin a foam layer composition can be about 1-20 wt. %, about 2-15 wt. %,about 5-10 wt. %, or about 6-8 wt. % of the foam layer composition. Insome embodiments, the amount of chemical foaming agent in a cap layercomposition can be about 0.1-5 wt. %, about 0.5-3 wt. %, or about 1-2wt. % of the cap layer composition. The amount of chemical foaming agentcan depend on the unfoamed sheet thickness, desired foam thickness,desired foam density, materials being extruded, crosslinking percentage,type of chemical foaming agent (different foaming agents can generatesignificantly different quantities of gas), among others.

Note that the above listed amounts of chemical foaming agent can bespecific to ADCA only. Other foaming agents can produce varying amountsof volumetric gas per mass of CFA and can be considered accordingly. Forexample, when comparing ADCA to the chemical foaming agentp-toluenesulfonyl semicarbazide (TSS), if a foamable layer contains 40PPHR ADCA, about 63 PPHR TSS would be required to generate about thesame amount gas during the foaming step.

If the difference between the decomposition temperature of the thermallydecomposable foaming agent and the melting point of the polymer with thehighest melting point is high, then a catalyst for foaming agentdecomposition may be used. Exemplary catalysts include, but are notlimited to, zinc oxide, magnesium oxide, calcium stearate, glycerin, andurea. The lower temperature limit for extrusion can be that of thepolymer with the highest melting point. If the extrusion temperaturedrops below the melting temperature of the polymer with the highestmelting point, then undesirable “unmelts” appear. Upon foaming, theextruded layer that was extruded below this lower temperature limit canexhibit uneven thickness, a non-uniform cell structure, pockets of cellcollapse, and other undesirable attributes.

Extruding an unfoamed multilayer sheet and extruding a foamed multilayersheet (commonly referred to as “extrusion foaming”) are vastlydifferent. Extrusion foaming can be performed with a physical foamingagent, a chemical foaming agent, or a mixture of physical and chemicalfoaming agents. Physical foaming agents can be inorganic and organicgases (nitrogen, carbon dioxide, pentane, butane, etc.) that areinjected under high pressure directly into the polymer melt. The gasescan nucleate and expand as the polymer melt exits the extrusion die tocreate the foamed polymer. Chemical foaming agents—such as the examplespreviously described—can be solids that decompose exothermally orendothermally upon a decomposition temperature to produce gases. Typicalgases generated from chemical foaming agents include nitrogen, carbonmonoxide, carbon dioxide, ammonia, etc. To extrusion foam a chemicalfoaming agent, the chemical foaming agent can be dispersed in thepolymer melt and the melt heated to above the decomposition temperatureof the chemical foaming agent while still in the extruder and die. Afoamed polymer can be made as the polymer melt exits the extrusion die.

Regardless of whether the foaming agents are physical, chemical, or acombination, typical extrusion foaming generates polymer sheets whereboth primary surfaces are significantly rougher than equivalentstructures produced in the disclosed method. The surface profile of amultilayer (as well as a single layer) foam sheet can be critical inmany applications and thus extrusion foamed sheets may not be used forthese applications. These applications can require a smooth foam surfaceto obtain desired properties such as ease of lamination to a film,fabric, fiber layer, and a leather; percentage contact area in thelamination; visual aesthetics; etc. PCT Publication WO 2016109544, whichis hereby incorporated in its entirety by reference, includes examplesillustrating the difference in surface roughness between extrusionfoamed polymer sheets and equivalent foamed polymer sheets produced bythe disclosed method.

The rougher surfaces of extrusion foamed articles can be generallycaused by larger sized cells (when compared to the foams producedaccording to the present disclosure). Although the cell size and cellsize distribution may not be as critical in most commercialapplications, because surface roughness is a function of cell size,foams with larger cells can be less desirable than foams with smallercells for applications requiring a smooth foam surface.

The thickness of the unfoamed, coextruded multilayer structure can beabout 0.1 to about 30 mm, about 0.2 to about 25 mm, about 0.3 to about20 mm, or about 0.4 to about 15 mm. Any individual A or B layer can havea thickness of at least about 0.05 mm, at least about 0.1 mm, at leastabout 0.15 mm, or at least about 0.2 mm. Any individual A or B layer canhave a thickness of less than or equal to about 0.2 mm, about 0.15 mm,or about 0.10 mm. In some embodiments, a cap layer of the unfoamed,coextruded multilayer structure can have a thickness of about 0.1-300microns, about 25-200 microns, or about 30-175 microns. In someembodiments, a cap layer of the unfoamed, coextruded multilayerstructure can have a thickness of less than 300 microns, less than 250microns, less than 200 microns, less than 175 microns, less than 150microns, less than 125 microns, less than 100 microns, less than 90microns, less than 80 microns, less than 70 microns, less than 60microns, less than 50 microns, less than 40 microns, less than 30microns, less than 20 microns, less than 10 microns, less than 5microns, or less than 1 micron. In some embodiments, a cap layer of theunfoamed, coextruded multilayer structure can have a thickness of morethan 1 micron, more than 5 microns, more than 10 microns, more than 20microns, more than 30 microns, more than 40 microns, more than 50microns, more than 60 microns, more than 70 microns, more than 80microns, more than 90 microns, more than 100 microns, more than 125microns, more than 150 microns, more than 175 microns, more than 200microns, or more than 250 microns. The unfoamed cap thickness is notlimited in how thin it can be in relation to the overall unfoamedcoextruded multilayered sheet, and may be as thin as about 0.1 μm, orthe typical thickness of a very thin tie layer used in multilayeredflexible packaging and barrier films. In some embodiments, a foam layerof the unfoamed, coextruded multilayer structure can have a thickness ofabout 0.1-5 mm, about 0.5-3 mm, about 1-2 mm, or about 1-1.5 mm. In someembodiments, a foam layer of the unfoamed, coextruded multilayerstructure can have a thickness of less than or equal to about 5 mm,about 3 mm, about 2 mm, about 1.5 mm, about 1 mm, or about 0.5 mm. Insome embodiments, a foam layer of the unfoamed, coextruded multilayerstructure can have a thickness of greater than or equal to about 0.1 mm,about 0.5 mm, about 1 mm, about 1.5 mm, about 2 mm, or about 3 mm.

For cases where the cap layers are either not intended to be foamed orare only lightly foamed, the cap can be thin and easily pliable whenmelted so as to not significantly hinder the expansion of the foamablelayer(s) during the foaming step. The cap's thickness, flexibility, meltstrength, and crosslinking percentage are among many physical propertiesthat can hinder the foaming expansion of the other layer(s). Similarly,the thickness, flexibility, melt strength, and crosslinking percentageof the foamable layer(s) as well as the ultimate thickness and densityof the foamed layers are also factors in whether the cap inhibits theexpansion of the foamable layer(s). A general guideline for maximum capthickness is that it should be no more than about 20%, about 15%, about10%, or about 5% of the overall coextruded unfoamed sheet. If the capthickness is greater than about 20% of the overall coextruded unfoamedsheet, problems with the multilayered sheet curling, buckling, andfolding onto itself may occur as the multilayered sheet is heated andfoamed.

After the coextruded sheet has been produced (e.g., by two extruders),the extruded multilayered sheet can be subjected to irradiation withionizing radiation at a given exposure to crosslink the composition ofthe multilayered sheet, thereby obtaining an irradiated, crosslinkedmultilayer structure. Ionizing radiation is often unable to produce asufficient degree of crosslinking on polypropylene(s),polypropylene-based materials, some polyethylene(s), and somepolyethylene-based materials. Thus, a crosslinking promoter can be addedto the compositions that are fed into the extruders to promotecrosslinking. Polymers crosslinked by ionizing radiation are commonlyreferred to as “physically crosslinked”.

It is important to distinguish between “physical” crosslinking and“chemical” crosslinking. In chemical crosslinking, the crosslinks aregenerated with crosslinking promoters but without the use of ionizingradiation. Chemical crosslinking typically involves using peroxides,silanes, or vinylsilanes. In peroxide crosslinking processes, thecrosslinking typically occurs in the extrusion die. For silane andvinylsilane crosslinking processes, the crosslinking typically occurspost-extrusion in a secondary operation where the crosslinking of theextruded material is accelerated with heat and moisture. Regardless ofthe chemical crosslinking method, chemically crosslinked foam sheetstypically exhibit primary surfaces that are significantly rougher thanequivalent structures produced in the disclosed method. The surfaceprofile of a multilayer (as well as single layer) foam sheet can becritical in many applications and thus chemically crosslinked foamsheets may not be used for these applications. These applications canrequire a smooth foam surface to obtain desired properties such as easeof lamination to a film, fabric, fiber layer, and a leather; percentagecontact area in the lamination; visual aesthetics; etc. PCT PublicationWO 2016109544 includes examples illustrating the difference in surfaceroughness between chemically crosslinked foamed polymer sheets andequivalent foamed polymer sheets produced by the disclosed method.

The rougher surfaces of chemically crosslinked foamed articles can begenerally caused by larger sized cells (when compared to the foamsproduced according to the present disclosure). Although the cell sizeand size distribution is not critical in most commercial applicationsbecause surface roughness is a function of cell size, foams with largercells can be less desirable than foams with smaller cells forapplications requiring a smooth foam surface.

Examples of ionizing radiation include, but are not limited to, alpha,beta (electron beams), x-ray, gamma, and neutron. Among them, anelectron beam having uniform energy can be used to prepare thecrosslinked polyolefin foam/KEE cap structure. Exposure time, frequencyof irradiation, and acceleration voltage upon irradiation with anelectron beam can vary widely depending on the intended crosslinkingdegree and the thickness of the multilayered structure. However, theionizing radiation can generally be in the range of from about 10 toabout 500 kGy, about 20 to about 300 kGy, or about 20 to about 200 kGy.If the exposure is too low, then cell stability may not be maintainedupon foaming. If the exposure is too high, the moldability of theresulting multilayered foam structure may be poor. Moldability is adesirable property when the multilayered foam sheet is used inthermoforming applications. Also, the unfoamed sheet may be softened byexothermic heat release upon exposure to the electron beam radiationsuch that the structure can deform when the exposure is too high. Inaddition, the polymer components may also be degraded from excessivepolymer chain scission.

The coextruded unfoamed multilayered sheet may be irradiated up to fourseparate times, preferably no more than twice, and more preferably onlyonce. If the irradiation frequency is more than about four times, thepolymer components may suffer degradation so that upon foaming, forexample, uniform cells will not be created in the resulting foamlayer(s). When the thickness of the extruded structure is greater thanabout 4 mm, irradiating each primary surface of the multilayered profilewith an ionized radiation can be preferred to make the degree ofcrosslinking of the primary surface(s) and the inner layer more uniform.

Irradiation with an electron beam provides an advantage in thatcoextruded sheets having various thicknesses can be effectivelycrosslinked by controlling the acceleration voltage of the electrons.The acceleration voltage can generally be in the range of from about 200to about 1500 kV, about 400 to about 1200 kV, or about 600 to about 1000kV. If the acceleration voltage is less than about 200 kV, then theradiation may not reach the inner portion of the coextruded sheets. As aresult, the cells in the inner portion can be coarse and uneven onfoaming. Additionally, acceleration voltage that is too low for a giventhickness profile can cause arcing, resulting in “pinholes” or “tunnels”in the foamed structure. On the other hand, if the acceleration voltageis greater than about 1500 kV, then the polymers may degrade. In someembodiments, the radiation source may face the B layer of thecoextruded, unfoamed multilayer sheet during irradiation. In someembodiments, the radiation source may face the A layer of thecoextruded, unfoamed multilayer sheet during irradiation.

Regardless of the type of ionizing radiation selected, crosslinking isperformed so that the composition of the extruded structure iscrosslinked about 20 to about 75% or about 30 to about 60%, as measuredby the “Toray Gel Fraction Percentage Method.” According to the “TorayGel Fraction Percentage Method,” tetralin solvent is used to dissolvenon-crosslinked components in a composition. In principle, thenon-crosslinked material is dissolved in tetralin and the crosslinkingdegree is expressed as the weight percentage of crosslinked material inthe entire composition. The apparatus used to determine the percent ofpolymer crosslinking includes: 100 mesh (0.0045 inch wire diameter);Type 304 stainless steel bags; numbered wires and clips; a Miyamotothermostatic oil bath apparatus; an analytical balance; a fume hood; agas burner; a high temperature oven; an anti-static gun; and three 3.5liter wide mouth stainless steel containers with lids. Reagents andmaterials used include tetralin high molecular weight solvent, acetone,and silicone oil. Specifically, an empty wire mesh bag is weighed andthe weight recorded. For each sample, 100 milligrams ±5 milligrams ofsample is weighed out and transferred to the wire mesh bag. The weightof the wire mesh bag and the sample, typically in the form of thinlysliced foam cuttings, is recorded. Each bag is attached to thecorresponding number wire and clips. When the solvent temperaturereaches 130° C., the bundle (bag and sample) is immersed in the solvent.The samples are shaken up and down about 5 or 6 times to loosen any airbubbles and fully wet the samples. The samples are attached to anagitator and agitated for three (3) hours so that the solvent candissolve the foam. The samples are then cooled in a fume hood. Thesamples are washed by shaking up and down about 7 or 8 times in acontainer of primary acetone. The samples are washed a second time in asecond acetone wash. The washed samples are washed once more in a thirdcontainer of fresh acetone as above. The samples are then hung in a fumehood to evaporate the acetone for about 1 to about 5 minutes. Thesamples are then dried in a drying oven for about 1 hour at 120° C. Thesamples are cooled for a minimum of about 15 minutes. The wire mesh bagis weighed on an analytical balance and the weight is recorded.Crosslinking is then calculated using the formula 100*(C−A)/(B−A), whereA=empty wire mesh bag weight; B=wire bag weight+foam sample beforeimmersion in tetralin; and C=wire bag weight+dissolved sample afterimmersion in tetralin.

Suitable crosslinking promoters include, but are not limited to,commercially available difunctional, trifunctional, tetrafunctional,pentafunctional, and higher functionality monomers. Such crosslinkingmonomers are available in liquid, solid, pellet, and powder forms.Examples include, but are not limited to, acrylates or methacrylatessuch as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate,ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, tetramethylol methane triacrylate,1,9-nonanediol dimethacrylate and 1,10-decanediol dimethacrylate; allylesters of carboxylic acid (such as trimellitic acid triallyl ester,pyromellitic acid triallyl ester, and oxalic acid diallyl ester); allylesters of cyanulic acid or isocyanulic acid such as triallyl cyanurateand triallyl isocyanurate; maleimide compounds such as N-phenylmaleimide and N,N′-m-phenylene bismaleimide; compounds having at leasttwo tribonds such as phthalic acid dipropagyl and maleic aciddipropagyl; and divinylbenzene. Additionally, such crosslinkingpromoters may be used alone or in any combination. Divinylbenzene (DVB),a difunctional liquid crosslinking monomer, can be used as acrosslinking promoter in the present disclosure. For example, a suitablecommercially-available DVB may include DVB HP by the Dow ChemicalCompany.

The amount of crosslinking promoter in a foam layer composition and/or acap layer composition can be less than or equal to about 5 PPR %, about4 PPR %, about 3 PPR %, about 2.5 PPR %, about 2 PPR %, about 1.5 PPR %,or about 1 PPR % of the composition. In some embodiments, the amount ofcrosslinking promoter in a foam layer composition and/or a cap layercomposition can be greater than or equal to about 0.5 PPR %, about 1 PPR%, about 1.5 PPR %, about 2 PPR %, about 2.5 PPR %, about 3 PPR %, orabout 4 PPR % of the composition. In some embodiments, the amount ofcrosslinking promoter in a foam layer composition and/or a cap layercomposition can be about 0.1-5 PPR %, about 0.5-3 PPR %, about 1-3 PPR%, or about 2-3 PPR % of the composition. In some embodiments, theamount of crosslinking promoter in a foam layer composition can be about0.5-5 wt. % or about 1-3 wt. % of the foam layer composition.

Note that the above listed amounts of crosslinking promoter can bespecific to DVB only. Other crosslinking promoters can be more or lessefficient in crosslinking than DVB. Thus, the required quantity foranother crosslinking promoter should be considered accordingly.Crosslinking promoters can vary in crosslinking efficiency based on theionizing radiation dosage, the polymers being crosslinked, the chemicalstructure of the monomer, the number of functional groups on themonomer, and whether the monomer is a liquid or a powder.

Crosslinks may be generated using a variety of different techniques andcan be formed both intermolecularly, between different polymermolecules, and intramolecularly, between portions of a single polymermolecule. Such techniques include, but are not limited to, providingcrosslinking promoters which are separate from a polymer chain andproviding polymer chains which incorporate a crosslinking promotercontaining a functional group which can form a crosslink or be activatedto form a crosslink.

After irradiating the coextruded sheet, foaming may be accomplished byheating the crosslinked multilayered sheet to a temperature higher thanthe decomposition temperature of the thermally decomposable blowingagent. The foaming can be performed at about 200-260° C. or about220-240° C. in a continuous process. A continuous foaming process can bepreferred over a batch process for production of a continuous foamsheet.

The foaming can be typically conducted by heating the crosslinkedmultilayered sheet with molten salt, radiant heaters, vertical orhorizontal hot air oven, microwave energy, or a combination of thesemethods. The foaming may also be conducted in an impregnation processusing, for example, nitrogen in an autoclave, followed by a free foamingvia molten salt, radiant heaters, vertical or horizontal hot air oven,microwave energy, or a combination of these methods. Optionally, beforefoaming, the crosslinked multilayered sheet can be softened withpreheating. This can help stabilize the expansion of the structure uponfoaming, particularly with thick and stiff sheets.

The density of the multilayered foam sheet can be defined and measuredusing section or “overall” density, rather than a “core” density, asmeasured by JIS K6767. The multilayered foam sheets produced using theabove described method can yield foams with a section, or “overall”density of about 20-250 kg/m³, about 30-200 kg/m³, or about 50-150kg/m³. The section density can be controlled by the amount of blowingagent and the thickness of the extruded structure. If the density of themultilayered foam sheet is less than about 20 kg/m³, then the sheet maynot foam efficiently due to a large amount of chemical blowing agentneeded to attain the density. Additionally, if the density of the sheetis less than about 20 kg/m³, then the expansion of the sheet during thefoaming step may become increasingly difficult to control. Furthermore,if the density of the multilayered foam sheet is less than about 20kg/m³, then the foam may become increasingly prone to cell collapse.Thus, it may be difficult to produce a multilayered foam sheet ofuniform section density and thickness at a density less than about 20kg/m³.

The multilayered foam sheet is not limited to a section density of about250 kg/m³. A foam having a section density of about 350 kg/m³, about 450kg/m³, or about 550 kg/m³ may also be produced. However, it may bepreferred that the foam sheet have a density of less than about 250kg/m³ since greater densities can be generally cost prohibitive whencompared to other materials which can be used in a given application.

The foam layers produced using the above method may have closed cells.Preferably, at least 90% of the cells have undamaged cell walls,preferably at least 95%, and more preferably more than 98%. The averagecell size can be from about 0.05 to about 1.0 mm, and preferably fromabout 0.1 to about 0.7 mm. If the average cell size is lower than about0.05 mm, then the density of the foam structure can typically be greaterthan 250 kg/m³. If the average cell size is larger than 1 mm, the foammay have an uneven surface. There is also a possibility of the foamstructure being undesirably torn if the population of cells in the foamdoes not have the preferred average cell size. This can occur when thefoam structure is stretched or portions of it are subjected to asecondary process. The cell size in the foam layer(s) may have a bimodaldistribution representing a population of cells in the core of the foamstructure which are relatively round and a population of cells in theskin near the surfaces of the foam structure which are relatively flat,thin, and/or oblong.

The overall thickness of the multilayered polyolefin foam/KEE cap sheetcan be about 0.2 mm to about 50 mm, about 0.4 mm to about 40 mm, about0.6 mm to about 30 mm, or about 0.8 mm to about 20 mm. If the thicknessis less than about 0.2 mm, then foaming may not be efficient due tosignificant gas loss from the primary surface(s). If the thickness isgreater than about 50 mm, expansion during the foaming step can becomeincreasingly difficult to control. Thus, it can be increasingly moredifficult to produce a multilayered polyolefin foam/KEE cap sheet withuniform section density and thickness. In some embodiments, a cap layerof the foamed, coextruded multilayer structure can have a thickness ofabout 0.1-100 microns, about 1-100 microns, or about 5-75 microns. Insome embodiments, a foam layer of the foamed, coextruded multilayerstructure can have a thickness of about 0.1-5 mm, about 0.5-5 mm, about1-5 mm, about 2-5 mm, or about 2-4 mm.

In some embodiments, the desired thickness can be obtained by asecondary process such as slicing, skiving, or bonding. Slicing,skiving, or bonding can produce a thickness range of about 0.1 mm toabout 100 mm.

For the cap layer(s) intended to be unfoamed or lightly foamed, thethickness of the cap layer may be reduced upon foaming of themultilayered sheet. This can be due to the foamable layer(s) expandingand consequently stretching the cap layer(s). Thus, for example, if themultilayered sheet expands to twice its original area, the cap thicknesscan be expected to be about halved. If the multilayered sheet expands tofour times its original area, the cap thickness can be expected to bereduced to about one-quarter of its original thickness.

The disclosed multilayered polyolefin foam/KEE cap sheets can be usedfor applications where adhesion to PVC is required. The PVC can be aflexible film, foil, or foam. The PVC can also be a semi-flexible orrigid board, plank, tile, or substrate. The board, plank, tile, orsubstrate can be solid or foam. Importantly, the PVC may be a surfacelayer of a multilayered structure.

In some embodiments, the multilayer foam structures are laminatescontaining the multilayer foam and a flexible laminate layer.Preferably, the laminate layer can be applied to the KEE cap side of themultilayer foam. In these laminates, the multilayer foam structure can,for example, can be combined with a film, foil, or foam. Examples ofsuitable materials for such laminate layers include, but are not limitedto, flexible PVC films, flexible PVC foils, and flexible PVC foams. Suchlayers may be manufactured using standard techniques that are well knownto those of ordinary skill in the art. Importantly, the multilayer foamof the disclosure may be laminated on one or both sides with thesematerials and may include multiple other layers. If the multilayer foamis laminated on both sides, preferably these laminate layers can beapplied to KEE cap layers of the multilayer foam.

The multilayer foam structures (or laminates comprising the multilayeredfoam structures) can also be thermoformed. To thermoform the multilayerfoam structure or laminate, the foam can be heated to the melting pointof the blend for all the layers in the multilayer foam/KEE capstructure. If any layer has immiscible polymers, the multilayer foamstructure may exhibit more than one melting point. In this case, themultilayer foam structure can typically be thermoformed when the foam isheated to a temperature midway between the multilayer foam layercomposition's lowest melting point and highest melting point. Inaddition, the multilayer foam structure can be thermoformed onto asubstrate such as a hard polypropylene, ABS, or wood fiber composite. Aheat activated adhesive may be used to improve the bonding of thesubstrate to a capped or uncapped side of the multilayer foam/KEE capstructure. In cases of a laminate, the laminate layer may be on theopposite side of the substrate (for example, in cases where the laminatelayer is for a protective and/or decorative purpose.) The substrateitself can also be thermoformed at the same time as the multilayer foamstructure.

In some embodiments, the multilayer foam structures or laminates (whichmay or may not be thermoformed) can be used in automobile interior partssuch as door panels, door rolls, door inserts, door stuffers, trunkstuffers, armrests, center consoles, seat cushions, seat backs,headrests, seat back panels, instrument panels, knee bolsters, or aheadliner. These multilayer foam structures or laminates (which may ormay not be thermoformed) can also be used in furniture (e.g.,commercial, office, and residential furniture) such as chair cushions,chair backs, sofa cushions, sofa trims, recliner cushions, reclinertrims, couch cushions, couch trim, sleeper cushions, or sleeper trims.These multilayer foam laminates or structures (which may or may not bethermoformed) can also be used in walls such as modular walls, moveablewalls, wall panels, modular panels, office system panels, room dividers,or portable partitions. The multilayer foam laminates or structures canalso be used in storage casing (e.g., commercial, office andresidential) which can be either mobile or stationary. Furthermore, themultilayer foam laminates and structures (which may or may not bethermoformed) can also be used in coverings such as chair cushioncoverings, chair back coverings, armrest coverings, sofa coverings, sofacushion coverings, recliner cushion coverings, recliner coverings, couchcushion coverings, couch coverings, sleeper cushion coverings, sleepercoverings, wall coverings, and architectural coverings.

In some embodiments, the multilayer foam structures or laminates can beused as roofing membranes and as a component in roof waterproofing. Forlaminates: PVC, KEE, and PVC/KEE blends are well suited as the laminatelayer to the multilayer foam structure. The KEE cap layer of themultilayer foam structure may face the laminate layer.

In some embodiments, the multilayer foam structures are laminatescontaining the multilayer foam and a non-flexible laminate layer.Preferably, the multilayer foam can be applied with the KEE cap side tothe laminate layer. In these laminates, the multilayer foam structurecan, for example, be combined with a board, plank, tile, or substrate.The board, plank, tile, or substrate may be a foam. Such layers may bemanufactured using standard techniques that are well known to those ofordinary skill in the art. Importantly, the multilayer foam of thedisclosure may be laminated on one or both sides with these materialsand may include multiple other layers. If the multilayer foam islaminated on both sides, preferably these laminate layers can be appliedto cap layers of the multilayer foam.

Some embodiments include a first layer of the disclosed multilayer foamstructure and a second layer consisting of either a vinyl floor tile ora wood-PVC composite for flooring or walls. In these laminates, thefirst layer may be joined to the adjacent tile or composite by meltbonding. Preferably, the KEE cap layer of the multilayer foam structuremay on the side of the first layer facing the second layer

The multilayer foam attached to a vinyl floor tile or a wood-PVCcomposite can serve several purposes. The foam can reduce the reflectedsound pressure level when the panel is impacted, for example, whenwalking on the panel with boots or high heeled shoes. The foam can alsoact as a moisture vapor barrier between the panel and sub-floor(plywood, oriented strandboard (OSB), concrete, etc.) and can helpprovide a more uniform laydown among multiple panels since anyunevenness, bumps, or spikes (for example a protruding nailhead) on thesub-floor will be buffered by the foam. These tiles and composites arecommonly installed in residential homes, office buildings, and othercommercial buildings.

To satisfy the requirements of any of the above applications, thedisclosed structures of the present disclosure may be subjected tovarious secondary processes, including and not limited to, embossing,corona or plasma treatment, surface roughening, surface smoothing,perforation or microperforation, splicing, slicing, skiving, layering,bonding, and hole punching.

Examples Raw Materials for Examples

The following Table 1 provides a list of various components anddescriptions of those components used in the following Examples.

TABLE 1 Melt Flow Description/ Component Type Manufacturer Index Notes6232 PP/PE Total 1.3-1.6 commercially random Petrochemicals (2.16produced copolymer kg, 230° C.) Infuse ™ OBC Dow Chemical 0.75-1.25commercially OBC (PE/octene Company (2.16 kg, produced olefin 9107copolymer 190° C.) block copolymer based) Adflex ™ rTPO LyondellBasell0.5-0.7 commercially Q100F (PP/PE (2.16 produced random kg, reactorcopolymer 230° C.) thermoplastic based) polyolefin Plexar ® MAH-g-LyondellBasell nominal commercially PX6006 PP/PE 4.0 produced random(2.16 kg, maleic copolymer 230° C.) anhydride grafted polypropylene-polyethylene random copolymer LLP8501.67 LLDPE ExxonMobil 5.9-7.5commercially (LLDPE/ (2.16 produced hexene kg, copolymer) 190° C.)Elvaloy ® KEE Dow Chemical nominal commercially HP441 (ketone/ Company 8produced ethylene/ (2.16 kg, “KEE”: ester 190° C.) E/nBA/CO =terpolymer) ethylene/n-butyl acrylate/carbon monoxide terpolymerElvaloy ® KEE Dow Chemical nominal commercially HP641 (ketone/ Company12 produced ethylene/ (2.16 kg, “KEE”: ester 190° C.) E/nBA/CO =terpolymer) ethylene/n-butyl acrylate/carbon monoxide terpolymerAzofoam ® chemical P.T. Lauten — commercially TC-18I foaming Otsukaproduced agent Chemical azodicar- (ADCA) bonamide DVB HP crosslinkingDow Chemical — commercially promoter Company produced, 80% DVB content“PR023” anti-oxidant Techmer PM — a Toray Plastics package (America)(LDPE standard carrier) antioxidant package for polyolefin foam,compounded by Techmer PM, consisting of 14% antioxidants, 0.35% calciumstearate, and 85.65% low density polyethylene (LDPE) carrier resin“PR086” anti-oxidant Amfine — a Toray Plastics package Chemical(America) standard antioxidant package for polyolefin foam, 100%antioxidant powders TPM11166 processing Techmer PM — commercially aidproduced (LLDPE/ extrusion butane processing copolymer aid blendcarrier) PM91399 black Techmer PM — commercially concentrate produced(LDPE concentrate, carrier) 10% carbon black loading, 27 Nm typicalcarbon black particle size

Conversion Process for Examples

The following Table 2 provides the formulations for Examples 1-11.

FORMULATIONS resins (PPR % & overall %) additives (PPR % & overall %)OBC rTPO LLDPE KEE KEE chemical anti- (PE/octene (PP/PE randomMAH-g-PP/PE (PE/hexene (ketone/ (ketone/ foaming oxidant processing aidPP-PE copolymer copolymer random random ethylene/ester ethylene/esteragent x-linking package anti- (LLDPE/butene black random based) based)copolymer copolymer terpolymer) terpolymer) (ADCA) promoter (LDPEoxidant copolymer concentrate copolymer Infuse ™ Adflex ™ Plexar ®based) Elvaloy ® Elvaloy ® Azofoam ® DVB carrier) package carrier) (LDPEcarrier) example ID layer ID 6232 OBC 9107 Q100F PX6006 LLP8501.67 HP441HP661 TC-18I HP “PR023” “PR086” TPM11166 PM91399 Examples “B” cap 75.90%24.10% 0.58% 1.20% 1A and 1B layer 74.57% 23.67% 0.57% 1.18% “A” layer  50%   40%   10% 6.75%  2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57%5.78% 2.14% 4.71% 1.71% Examples “B” cap 51.22% 48.78% 0.59% 2.44% 2Aand 2B layer 49.71% 47.35% 0.57% 2.37% “A” layer   50%   40%   10% 6.75% 2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57% 5.78% 2.14% 4.71% 1.71%Example 3 “B” cap 25.93% 73.89% 0.60% 3.70% layer 24.90% 70.97% 0.58%3.56% “A” layer   50%   40%   10% 6.75%  2.5%  5.5%   2% (foamed) 42.83%34.26% 8.57% 5.78% 2.14% 4.71% 1.71% Examples “B” cap 75.90% 24.10%0.58% 1.20% 4A and 4B layer 74.57% 23.67% 0.57% 1.18% “A” layer   50%  40%   10% 6.75%  2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57% 5.78%2.14% 4.71% 1.71% Example 5 “B” cap 51.22% 48.78% 0.59% 2.44% layer49.71% 47.35% 0.57% 2.37% “A” layer   50%   40%   10% 6.75%  2.5%  5.5%  2% (foamed) 42.83% 34.26% 8.57% 5.78% 2.14% 4.71% 1.71% Example 6 “B”cap 25.93% 73.89% 0.60% 3.70% layer 24.90% 70.97% 0.58% 3.56% “A” layer  50%   40%   10% 6.75%  2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57%5.78% 2.14% 4.71% 1.71% Example 7 “B” cap 51.22% 48.78% 1.54% 0.59%2.44% layer 48.98% 46.65% 1.47% 0.57% 2.33% (foamed) “A” layer   50%  40%   10% 6.75%  2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57% 5.78%2.14% 4.71% 1.71% Example 8 “B” cap 51.22% 48.78% 0.59% 2.44% layer49.71% 47.35% 0.57% 2.37% “A” layer   50% 36.36% 9.09%  4.55% 6.75% 2.5%  5.5%   2% (foamed) 42.83% 31.15% 7.79%  3.89% 5.78% 2.14% 4.71%1.71% Example 9 “B” cap   25%   75% 0.58% 1.25% layer 24.55% 73.65%0.57% 1.23% “A” layer   50%   40%   10% 6.75%  2.5%  5.5%   2% (foamed)42.83% 34.26% 8.57% 5.78% 2.14% 4.71% 1.71% Example 10 “B” cap   50%  50% 0.58% 1.25% layer 49.10% 49.10% 0.57% 1.23% “A” layer   50%   40%  10% 6.75%  2.5%  5.5%   2% (foamed) 42.83% 34.26% 8.57% 5.78% 2.14%4.71% 1.71% Example 11 “B” cap   75%   25% 0.58% 1.25% layer 73.65%24.55% 0.57% 1.23% “A” layer   50%   40%   10% 6.75%  2.5%  5.5%   2%(foamed) 42.83% 34.26% 8.57% 5.78% 2.14% 4.71% 1.71%

The following Table 3 provides the coextrusion, irradiation, andproperties of the multilayer structure of Examples 1-11.

COEXTRUSION unfoamed IRRADIATION FOAMING specific overall unfoamed whichlayer heating via overall gel energy of sheet cap facing foaming moltenthickness (two checks, % extrusion melt temp. thickness thicknessradiation dosage voltage temp. salt or (cap = μm, overall density (TorayGel Fraction example ID layer ID type extruder (kW · hr/kg) (° C.) (mm)(μm) source? (kGy) (kV) (° F.) hot air? foam = mm) (kg/m³) PercentageMethod) Examples “B” cap 50/50 co- 0.20 177 1.49-1.55 50-140 towards50.2 725 428-433 molten 10-50 (Ex 1A), 122 (Ex 1A), 44, 45 1A and 1Blayer feed rotating IR salt 10-40 (Ex 1B)  75 (Ex 1B) block twinmanifold screw “A” layer co- 0.18 143 2.9 (Ex 1A), (foamed) rotating 3.2(Ex 1B) twin screw Examples “B” cap 50/50 co- 0.19 176 1.47-1.53 50-110towards 50.2 725 428-433 molten 20-50 (Ex 2A), 119 (Ex 2A) 43, 44 2A and2B layer feed rotating IR salt 10-40 (Ex 2B)  76 (Ex 2B) block twinmanifold screw “A” layer co- 0.18 143 2.9 (Ex 2A), (foamed) rotating 3.2(Ex 2B) twin screw Example 3 “B” cap 50/50 co- 0.19 176 1.45-1.50 50-110towards 50.2 725 428-433 molten  5-30 78 40, 44 layer feed rotating IRsalt block twin manifold screw “A” layer co- 0.18 143 3.0 (foamed)rotating twin screw Examples “B” cap 50/50 co- 0.20 176 1.56-1.60 50-130towards 50.2 725 428-433 molten 10-60 (Ex 4),  82 (Ex 4A), 46, 47 4A and4B layer feed rotating IR salt 10-50 (Ex 2B)  79 (Ex 4B) block twinmanifold screw “A” layer co- 0.18 143 3.1 (Ex 4A), (foamed) rotating 3.6(Ex 4B) twin screw Example 5 “B” cap 50/50 co- 0.19 176 1.53-1.59 50-130towards 50.2 725 428-433 molten 10-40 79 48, 49 layer feed rotating IRsalt block twin manifold screw “A” layer co- 0.18 143 3.4 (foamed)rotating twin screw Example 6 “B” cap 50/50 co- 0.19 175 1.48-1.5640-110 towards 50.2 725 428-433 molten 10-40 77 43, 44 layer feedrotating IR salt block twin manifold screw “A” layer co- 0.18 143 3.2(foamed) rotating twin screw Example 7 “B” cap 50/50 co- 0.21 1671.58-1.62 70-100 towards 50.2 725 428-433 molten  5-50 80 44, 48 layerfeed rotating IR salt block twin manifold screw 3.5 “A” layer co- 0.18143 (foamed) rotating twin screw Example 8 “B” cap 50/50 co- 0.23 1691.54-1.57 70-110 towards 50.2 725 428-433 molten 10-50 86 46, 47 layerfeed rotating IR salt block twin manifold screw “A” layer co- 0.17 1433.2 (foamed) rotating twin screw Example 9 “B” cap 50/50 co- 0.18 1761.56 60-80 towards 50.2 725 428-433 molten 10-40 97 44, 44 layer feedrotating IR salt block twin manifold screw “A” layer co- 0.17 144 3.0(foamed) rotating twin screw Example 10 “B” cap 50/50 co- 0.18 1761.52-1.56 40-100 towards 50.2 725 428-433 molten 10-40 83 45, 45 layerfeed rotating IR salt block twin manifold screw “A” layer co- 0.18 1433.1 (foamed) rotating twin screw Example 11 “B” cap 50/50 co- 0.18 1761.52-1.59 60-100 towards 50.2 725 428-433 molten 10-50 75 44, 45 layerfeed rotating IR salt block twin manifold screw “A” layer co- 0.18 1463.3 (foamed) rotating twin screw

Images of the multilayered structures of Examples 1A, 1B, 2A, 2B, 3, 4A,4B, 5, 6, 7, 8, 9, 10, and 11 at 30× magnification and 45° from primarysurface can be found in FIGS. 1A, 1B, 2A, 2B, 3, 4A, 4B, 5, 6, 7, 8, 9,10, and 11, respectively.

This application discloses several numerical ranges in the text andfigures. The numerical ranges disclosed inherently support any range orvalue within the disclosed numerical ranges, including the endpoints,even though a precise range limitation is not stated verbatim in thespecification because this disclosure can be practiced throughout thedisclosed numerical ranges.

The above description is presented to enable a person skilled in the artto make and use the disclosure, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the disclosure. Thus, this disclosure is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. Finally,the entire disclosure of the patents and publications referred in thisapplication are hereby incorporated herein by reference.

1. A method of forming a multilayer structure comprising: coextruding: afirst layer comprising: polypropylene, polyethylene, or a combination ofpolypropylene and polyethylene; a chemical foaming agent; and a secondlayer on a side of the first layer, the second layer comprising: atleast 15 wt. % ketone-ethylene-ester (KEE) terpolymer; andpolypropylene, polyethylene, or a combination of polypropylene andpolyethylene; irradiating the coextruded layers with ionizing radiation;and foaming the irradiated, coextruded layers.
 2. The method of claim 1,wherein the first layer comprises 2-15 wt. % KEE.
 3. The method of claim1, wherein the first layer comprises at least 70 wt. % polypropylene,polyethylene, or a combination of polypropylene and polyethylene.
 4. Themethod of claim 1, wherein the first layer comprises additives in anamount of 1-20 wt. %.
 5. The method of claim 1, wherein the second layercomprises additives in an amount of 1-8 wt. %.
 6. The method of claim 1,wherein the polypropylene has a melt flow index of 0.1-25 grams per 10minutes at 230° C.
 7. The method of claim 1, wherein the polyethylenehas a melt flow index of 0.1-25 grams per 10 minutes at 190° C.
 8. Themethod of claim 1, wherein the amount of chemical foaming agent in thefirst layer is 4-10 wt. %.
 9. The method of claim 1, wherein thechemical foaming agent comprises azodicarbonamide.
 10. The method ofclaim 1, wherein the first layer comprises a crosslinking agent.
 11. Themethod of claim 10, wherein the amount of crosslinking agent in thefirst layer is 1-3 wt. %.
 12. The method of claim 1, wherein theionizing radiation is selected from the group consisting of alpha, beta(electron beams), x-ray, gamma, and neutron.
 13. The method of claim 1,wherein the coextruded structure is irradiated up to four separatetimes.
 14. The method of claim 12, wherein the ionizing radiation is anelectron beam with an acceleration voltage of 200-1500 kV.
 15. Themethod of claim 14, wherein an absorbed electron beam dosage is 10-500kGy.
 16. The method of claim 1, wherein the ionizing radiationcrosslinks the extruded structure to a crosslinking degree of 20-75%.17. The method of claim 1, wherein foaming comprises heating theirradiated structure with molten salt and radiant heaters or a hot airoven.
 18. The method of claim 1, wherein the multilayer foam structurehas a density of 20-250 kg/m³.
 19. The method of claim 1, wherein themultilayer foam structure has an average closed cell size of 0.05-1.0mm.
 20. The method of claim 1, wherein the multilayer foam structure hasa thickness of 0.2-50 mm.